Field of the Invention
The invention concerns the determination of time-dependent dephasing factors of at least one spectral component from at least two spectral components in an examination region of an examination object by test measurements conducted with a magnetic resonance (MR) scanner, as well as a method for applying the dephasing factors that have been determined, and an MR apparatus and a non-transitory, computer-readable data storage medium for implementing such a method.
Description of the Prior Art
The magnetic resonance (MR) is a known modality by which it is possible to generate images of the inside of an examination object. Expressed in simple terms, for this purpose the examination object is positioned in a magnetic resonance scanner in a strong, static and homogenous basic magnetic field, also known as a B0 field, having field strengths of 0.2 Tesla to 7 Tesla and more, such that the nuclear spins of the object are oriented along the basic magnetic field. Radio-frequency excitation pulses (RF pulses) are directed into the examination object in order to trigger nuclear spin resonances. The nuclear spin resonances that have been triggered are measured as so-called k-space data, and this is used as the basis for reconstructing MR images or calculating spectroscopic data. For spatially encoding the measured data, rapidly switched magnetic gradient fields are superimposed on the basic magnetic field. The measured data that have been recorded are digitized and stored as complex numerical values in a so-called k-space matrix. An associated MR image can be reconstructed from the k-space matrix containing the values, e.g. by a multidimensional Fourier transformation.
In the context of MR measurements of nuclear spins, it is possible to separate fractions of different spectral components contained in the MR data. The spectral components represent different spin species, e.g. nuclear spins in a fat environment and a water environment, or even in a silicone environment. For this purpose, chemical-shift imaging multiecho MR measurement sequences are often used in the context of Dixon techniques. Such techniques typically make use of the effect that the resonance frequency of nuclear spins depends on the molecular and/or chemical environment. This effect is known as chemical shift. Different spin species therefore have different resonance frequencies which make up the measured spectrum of the MR data. For example, the difference between two resonance frequencies of different spectral components can be expressed in ppm (parts per million, i.e. 10−6).
Many chemical species (e.g. water) have mono-frequency MR spectra, while others (e.g. fat) have a non-mono-frequency MR spectrum. Others consist of multiple coupled resonances which have a known amplitude ratio, a known phase position (if applicable), and known frequency differences. This prior knowledge can be utilized when determining the overall signal of these species, see e.g. Provencher et al. “SW. Estimation of metabolite concentrations from localized in vivo proton NMR spectra” MRM 30: 672 (1993).
The MR signal from hydrogen nuclear spins in water is often considered to be a first fraction of the spectral component “water”, and from hydrogen nuclear spins in fatty acid chains to be a second fraction of the second spectral component “fat”. It is also possible in principle to examine further fractions of further spectral components. In the cited standard case, MR data can be used to determine a water MR image and a fat MR image, i.e. individual MR images of the two spectral components, respectively. This is relevant for a wide variety of clinical and/or medical applications.
In order to be able to separate the fractions of the various spectral components from each other, MR signals are captured at a number of echo times in the context of the Dixon technique. These MR signals together form the MR data. These different spectral components have different phase positions and amplitudes at the different echo times. Taking this effect into consideration, it is possible to determine the quantities of the chemical species separately.
A signal model that associates the measured or captured MR data with different physically relevant variables is generally used for this purpose. The different variables of the model represent the various fractions to be determined with respect to the spectral components, the spectra thereof and, depending on the accuracy, scope and complexity of the signal model, further unknowns of the measuring system. It is then possible for the fractions of the spectral components included in the signal model to be determined for each voxel of the MR data.
For example, the spectral model for fat as a spectral component is disclosed in Hamilton G. et al. “In vivo characterization of the liver fat 1H MR spectrum” NMR Biomed. 24: 784-790 (2011).
The results may vary according to the selected spectral model, however, since different assumptions are made in respect of the nature of the underlying spectrum of the fat in each case, and the fat which is actually present in the examination object may also be formed very differently depending on the examination object.
According to the procedure described by Hamilton et al., the fat spectra can be calibrated individually and then used as part of the signal model for Dixon techniques. However, this requires a considerable amount of time and a high degree of expertise and experience. Moreover, in the case of Dixon techniques using few echo times, the fat spectrum is only evaluated for correspondingly few complex-valued dephasing factors (phase position and amplitude) in the time window, and it is meanwhile possible to apply not only pure Dixon techniques using not only pure in-phase and opposed-phase contrasts.
DE102013217650A1 describes a method in which the dephasing factors are assumed to be known and the phase errors in magnetic resonance data records that were recorded by means of multi-contrast measurements are estimated.